1. Field of the Invention
The present invention relates to a satellite radio receiver that receives radio waves from positioning satellites to acquire satellite signals.
2. Description of Related Art
Some conventional portable devices such as electronic watches include receivers, which receive radio waves transmitted from positioning satellites belonging to the Global Navigation Satellite System (GNSS) including the Global Positioning System (GPS), to acquire time information and positional information. Such portable devices can correct the time to be displayed or can set a time zone based on the acquired time information and positional information.
The satellite signals output from the positioning satellites are transmitted as radio waves while being spread-modulated with unique spreading codes (pseudo-noise) set for each of the positioning satellites. Upon receiving the radio waves, the satellite signals from the positioning satellites can be decoded through inverse spreading using a spreading code set for each of the positioning satellites. A receiver that has no preliminary information on visibility or signal-receiving availability of the positioning satellites, which move on predetermined orbits, performs acquisition involving detection of satellite signals through inverse spreading on the received radio signals in a round robin fashion with spreading codes of all the positioning satellites so that an available positioning satellite is identified.
A matched filter (sliding correlator) has been used for such acquisition, and high-speed acquisition is achieved through concurrent inverse spreading with spreading codes of a plurality of satellites. Furthermore, Japanese Unexamined Patent Application Publication No. 2001-159670 discloses a technique that concurrently reads/writes data of concurrent inverse spreading from/to a plurality of memories to prevent a delay in I/O processing against data processing.
Japanese Unexamined Patent Application Publication No. 2002-122655 (corresponding to US 2002/0004392 A1) discloses a technique to set power and a clock signal to be supplied to a correlator for data acquisition and a microprocessor for computing the position or time based on acquired data to a required period and a required frequency, for the purpose of a reduction in power consumption.
The radio waves transmitted from the positioning satellite are received at a frequency different from the transmission frequency due to Doppler effect caused by the positioning satellite moving around the earth at high speed. Hence, in the case where the radio waves from the positioning satellite are received by a receiver having no current position data and no orbital data of the positioning satellites in advance, the receiver must search for a reception frequency of the radio waves from the positioning satellite within a possible variable frequency range due to the Doppler effect. This results in an increase in acquisition time in proportion to the number of reception frequency steps.
For example, in the matched filter, a storage unit for storing received data accounts for a large proportion of size compared with other components used for data acquisition in an acquisition circuit. Hence, if a plurality of storage units is provided for concurrently processing received data at various reception frequencies to achieve high-speed acquisition, the scale of the circuit for acquisition inconveniently increases.